1. Field of the Invention
The present invention relates to an image forming apparatus, a computer-readable storage medium, and an image forming method.
2. Description of the Related Art
As an example of an image forming apparatus such as a printer, a facsimile, a copying machine, a plotter, and MultiFunction Peripherals (MFP) including the foregoing, an inkjet recording apparatus and the like is known as a droplet ejecting recording type image forming apparatus using a recording head composed of a liquid ejecting head (droplet ejecting head) that ejects ink droplets. The liquid ejecting recording type image forming apparatus is an apparatus that ejects ink droplets from a recording head to a sheet conveyed (not restricted to paper, but meant to be a medium on which ink droplets or other liquid can adhere including an OHP transparency, and also referred to as a recording medium, recording paper, a recording sheet, and the like) to perform image formation (recording, printing, and imaging are also used as synonyms). Examples of the liquid ejecting recording type image forming apparatus include a serial type image forming apparatus in which a recording head ejects droplets while moving in a main-scanning direction to form an image and a line type image forming apparatus that uses a line head in which a recording head in a stationary condition ejects droplets to form an image.
In the present application, a liquid ejecting recording type image forming apparatus means an apparatus that carries out image formation by ejecting liquid to media such as paper, yarn, textile, fabric, leather, metal, plastic, glass, wood, and ceramics. Image formation means not only to give an image that has meaning of characters, graphics, and such to a medium, but also means to give an image that has no meaning such as patterns to a medium (simply making droplets land on a medium). Ink is not restricted to that referred to as ink, but is used as a generic name for all liquid that can perform image formation such as that referred to as recording liquid, fixing liquid, liquid, and the like, and includes, for example, DNA samples, resists, pattern material, and resin. An image is not limited to a planar image, but includes an image given to a three-dimensionally formed object and an image formed by three-dimensionally shaping a solid body.
In a serial type image forming apparatus, a carriage mounted with a recording head is moved for scanning in a main-scanning direction and a sheet that is a recording medium is conveyed intermittently, whereby an image is formed on the sheet. In such an image forming apparatus, when forming a color image by one main-scanning (one scan), at the time bidirectional printing that forms an image by reciprocating motions of the carriage is carried out, a color difference (bidirectional color difference) due to deviation in landing positions of droplets occurs. In addition, dot density becomes coarse because resolution relies on the pitch of a nozzle. As a result, image quality deteriorates.
Conventionally known are various technologies that achieve high image quality by making black color high in resolution only for color image formation (Japanese Patent Application Laid-open No. 2003-025614 and Japanese Patent Application Laid-open No. 2001-260423) and another technology that attempts to reduce bidirectional color difference by disposing nozzles that eject color droplets shifted in a nozzle array direction to unify the order of landing for bidirectional printing (Japanese Patent Application Laid-open No. 2004-106392).
Further known as a serial type image forming apparatus is an apparatus in which a black nozzle array arranged with a plurality of nozzles that eject black droplets and color nozzle arrays arranged with a plurality of nozzles that eject color droplets are disposed such that the black nozzle array is shifted by one head portion (for one nozzle array) in a sub-scanning direction downstream in a medium conveying direction with respect to the color nozzle arrays (Japanese Patent Application Laid-open No. 2010-208164).
Various arrangements are known for the head disposition to reduce bidirectional color difference mentioned above. Among them, the simplest configuration includes, for example, a plurality of heads that eject droplets of the same colors, with the heads being arranged in the reverse order in the same main-scanning direction as in the order of K, C, M, Y, M, C, and K, whereby different heads are used for out-bound and in-bound scans, and droplets are ejected constantly in the order of Y, M, C, and K so as to make the landing order of droplets of the respective colors the same for the out-bound and in-bound scans.
Adapting such a configuration reduces bidirectional difference. In addition, because a previously landed color becomes predominant, making black droplets land last can improve gray quality. In such a configuration, however, the number of heads is large.
Therefore, there is a need for an image forming apparatus capable of reducing bidirectional color difference with a simple head disposition.